The present invention relates to a fuel injection system for internal combustion engines, in which the number or occurences of fuel injection is variable dependent on the engine load.
Conventional fuel injection systems for multi-cylinder internal combustion engines are provided with solenoid-operated fuel injection valves respectively for the engine cylinders, the valves being simultaneously actuatable by a control circuit to inject fuel twice or more time per engine stroke for an improved engine response.
In a conventional four-cycle multi-cylinder engine, fuel has normally been injected twice an engine stroke. When the engine rotates at a speed of 6,000 rpm, one engine stroke takes 20 msec., and the maximum possible injection time required by each solenoid-operated valve is 10 msec. as the fuel is injected twice in 20 msec.
The solenoid-operated valve presently available operates such that its needle will not be lifted immediately upon application of a voltage, and will remain lifted for a certain period of time when the voltage is cut off before the valve is closed. When the voltage is applied again right after the solenoid-operated valve is de-energized, the valve will not be closed and be left substantially open. With the interval of time in which the valve is de-energized or at rest being too short, no linearity is maintained between the width of a voltage pulse applied to the valve and the amount of fuel injected under the influence of a previously applied voltage pulse. It is therefore required to give the valve an increased period of time in which the valve is to be de-energized. Such a period of time is normally required to be 1.5 msec., and hence the maximum controllable width of a pulse applied at the engine speed of 6,000 rpm is 8.5 msec.
Since the solenoid-operated valve will not be open immediately in response to application of a voltage, the minimum controllable width of a pulse to be applied to the valve has a limitation, and no proper adjustment of the rate of fuel flow is normally possible with a pulse width of 2 msec. or shorter.
For fuel economy, some internal combustion engines introduce a reduced amount of intake air while the engine is idling, and also deliver an increased amount of intake air while the engine is rotating at higher speeds through the use of a turbo charger. Such a system is required to have a greater ratio of the maximum to the minimum widths of pulses applied for fuel injection or a greater range in which the pulse width is variable in order to supply an amount of fuel in proportion to the amount of intake air introduced. However, the minimum pulse width is governed by the construction of solenoid-operated valves used and voltages applied thereto, and the maximum pulse width is limited for the reason described above.